This invention relates to a fault inspection apparatus and method for detecting a fault from the image picked up from the appearance of a specimen, or in particular to a fault inspection method for comparing the image of an object such as a semiconductor wafer, a TFT or a photomask obtained using the lamp light, laser beam or the electron beam with a reference image stored in advance and detecting a fine pattern fault or foreign matter.
The conventional technique for detecting a fault by comparing an image of an object of inspection (hereinafter referred to as the inspection object image) with a reference image is disclosed in JP-A-05-264467. In this conventional technique, images of specimens providing inspection objects having regularly arranged repetitive patterns are sequentially picked up and compared with images delayed in time by the repetitive pattern pitch so that an incoincident portion is detected as a pattern fault.
Actually, however, due to the vibration of a stage or the inclination of the object, the positions of the two images are not necessarily coincident with each other. Therefore, as disclosed in “Kensuke Takeda, Shun'ichi Kaneko, Takayuki Tanaka, Kaoru Sakai, Shunji Maeda, Yasuo Nakagawa: Robust Subpixel Image Alignment by Interpolation-based Increment Sign Matching, Proceedings of View 2004 of Workshop on Vision Technique Application, pp. 16-21, 2004” and “Kensuke Takeda, Shun'ichi Kaneko, Takayuki Tanaka, Kaoru Sakai, Shunji Maeda, Yasuo Nakagawa: Robust Subpixel Image Alignment by Interpolation-based Absolute Gradient Matching, Proceedings of the 11th Japan-Korea Joint Workshop on Frontiers of Computer Vision 2005 (FCV2005), pp. 154-159, 2005” the amount of displacement between the image picked up by the sensor and the image delayed by the repetitive pattern pitch is determined, and after setting the two images in position based on the displacement amount thus determined, the difference between the images is determined and, in the case where the difference is larger than a specified threshold value, a fault is determined, while in the case where the difference is smaller than the threshold, a non-fault, i.e. a normality is determined. This conventional inspection method is explained with the semiconductor wafer appearance inspection as an example. In the semiconductor wafer providing an object of inspection, as shown in FIG. 22A, a multiplicity of chips of the same pattern are arranged regularly. Each chip can be roughly classified into a memory mat portion 201 and a peripheral circuit portion 202 as shown in FIG. 22B. The memory mat portion 201 is a mass of small repetitive patterns (cells), while the peripheral circuit portion 202 is basically a mass of random patterns. Generally, the memory mat portion 201 is high in pattern density and the image obtained by a bright field illumination optical system is darkened. The peripheral circuit portion 202, on the other hand, is low in pattern density, and the image obtained is bright.
In the conventional appearance inspection, the images at the same positions of the adjacent chips such as the areas 222 and 223 in FIG. 22 are compared with each other in the peripheral circuit unit 202, and a portion where the brightness difference is larger than a threshold value is detected as a fault. This inspection method is hereinafter referred to as the chip comparison method. In the memory mat portion 201, on the other hand, the images of the adjacent cells are compared with each other, and a portion where the brightness difference is larger than a threshold value is detected as a fault. This inspection is hereinafter referred to as the cell comparison method.
Also, JP-A-2001-194323 discloses the coaxial epi-illumination/bright field detection method for radiating the DUV light or VUV light through an objective lens using a laser light source.